As is well known, the foundry art is that art dealing with the formation of metal articles by casting processes wherein molten metal is poured into a mold and the metal is cooled to allow it to solidify. By far the largest quantity of castings are made by processes in which the mold is formed from sand, i.e., by sand casting processes. There are several different sand casting processes, but the one employed most often is that employing green molding sand.
Green molding sand has been defined as a "plastic mixture of sand grains, clay, water and other materials which can be used for molding and casting processes. The sand is called `green` because of the moisture present and is thus distinguished from dry sand." (Heine et al., "Principles of Metal Casting," McGraw-Hill Book Co., Inc., New York (1955), p. 22). Green sand has also been defined as a "naturally bonded sand or a compounded molding-sand mixture which has been tempered with water for use while still in the damp or wet condition." ("Molding Methods and Materials," 1st Ed., The American Foundrymens' Society, Des Plaines (1962)). Such a sand contains water or moisture both in the mold-forming stage and in the metal casting phase. As employed herein, the term "foundry green molding sand" has reference to green molding sands of the type known to and employed by those of ordinary skill in the foundry art comprising molding sand and clay and tempered with water.
As is evident from the foregoing, the essential components of a foundry green molding sand are molding sand, clay and water. The molding sand, which usually is a silica sand (e.g., quartz), but which may be a zircon, olivine or other refractory particulate material having mesh sizes commonly in the range of from about 6 to about 270 mesh, serves largely as a filler and provides the body of the mold. The clay, which is a finely divided (normally less than about 2 microns) material such as montmorillonite (bentonite), illite, kaolinite, fire clay and the like, when plasticized with water, serves as a binder for the sand grains, and imparts the physical strength necessary to enable use of the green molding sand as a mold material. Ordinarily, green molding sands contain from about 5 to about 20 weight percent clay, based upon sand, and sufficient water, normally not greater than about 8 weight percent, based upon sand, to achieve the desired plasticity and other physical properties. The amount of temper water normally is greater when naturally-bonded sands are employed than when synthetic sands are employed.
There are a number of properties which are desired in foundry green molding sands. Among the most important are:
1. Good flowability or compactibility to permit easy pouring of the sand and to allow the sand to move against the pattern under compacting forces; PA1 2. Good physical strength after compaction to permit the mold to retain its shape after removal of the pattern and during casting; PA1 3. Dimensional stability during the casting process; PA1 4. Good internal cohesion of the sand grains and poor adhesion of the sand grains to the cast article; and PA1 5. Good collapsibility after casting to facilitate shakeout. PA1 1. The molding sand has good flowability. It is readily formed and compacted around mold patterns of complicated design. It can be readily employed in automatic molding machines. The good flowability permits achievement of a desired sand hardness and apparent bulk density with the expenditure of less compacting energy than with other green molding sands, and the danger of overramming a portion of the mold due to variations in compacting energy is reduced. PA1 2. The compacted sand possesses desirable green strength characteristics at lower moisture contents than are achieved with conventional green molding sands. PA1 3. The additive acts as a facing agent, and prevents burn-on or the fusing of quartz sand grains to the surface of the casting, and promotes excellent finish and peel. PA1 4. The additive reduces shifting of the sand during the casting process, whether it be mold wall movement or enlargement of the mold cavity, or whether it be a localized shifting of the sand resulting in such casting defects as rat-tails, scabs and buckles. PA1 5. The additive permits casting to be effected at lower pouring temperatures and promotes increased fluidity of the metal during casting. PA1 6. The additive yields adequate dry compression strength and yet excellent shakeout is obtained even with green molding sands employing Western bentonite as the clay binder. PA1 7. Finally, the additive is employed at relatively low levels, which in turn minimizes the formation of gas during casting.
There are, of course, subsidiary properties which are related to these properties, including compressive strength, permeability, compactibility, mold hardness, green shear, deformation, peel, and the like. In general, a green molding sand typically has properties within the following ranges:
______________________________________ Green Compression Strength 4-40 psi Green Shear Strength 0.5-10 psi Deformation 0.005-0.04 in/in Permeability 6.5-400 Dry Compression Strength 50-200+ psi Compactibility 35-65% ______________________________________
If the deformation or compactibility is too low, the green molding sand is too brittle and cannot withstand handling and pattern removal, while if the deformation is too high, dimensional accuracy cannot be maintained, and the mold, especially one of large mass, e.g., 100 pounds or more, may deform under its own weight. If both green strength and deformation are too high, the sand cannot be readily formed and compacted with existing technology. If permeability is less than 6.5, the vapors generated during casting cannot dissipate rapidly enough and the mold can rupture from gas pressure and molten metal can be ejected out of the sprues. If, on the other hand, the permeability is too high, the molten metal will not be retained in the mold cavity, but will penetrate the voids of the sand. Finally, if the dry strength is too low the sand cannot withstand the erosive effect of the flowing molten metal during casting, while if the dry strength is too high the casting may crack upon solidification.
Green molding sands may be referred to as "soft sands", because they remain plastic and re-formable throughout the mold forming procedure and, in part, during the casting operation. Such molding sands are quite distinct from other molding sands, which may be referred to as "hard sands". These "hard sands", although plastic at the beginning of the molding forming procedure, are hardened and become rigid prior to the casting operation. Hard sands are employed, for example, in investment molding processes, and in forming cores and molds made of resin-bonded sands, or sand formed of sodium silicate or phosphates, or baked drying oil sands. Such hardened sands have compression strengths of the order of 80 to 300 psi or higher. In contrast, green molding sands have compression strengths of the order of about 4 to about 40 psi and preferably about 12 to about 30 psi.
Green molding sands also may be distinguished from "hard sands" because they are readily recycled, it being necessary only to replace temper water and, if desired, organic or other additives lost during the casting process. In contrast, hard sands can be reclaimed only by removal of all materials except for the refractory grains, and complete replacement of the bonding material. As a consequence, hard sands commonly are discarded after one use.
Because of their quite different compositions and mode of use, the problems encountered in green sand casting procedures differ greatly from those of hard sand casting. One such problem is that of control of the amount of temper water to achieve adequate bond strength during both the forming and the casting steps. Slight changes in the amount of water in a green molding sand greatly affect the mechanical properties of the sand. In particular, the dry strength and the hot strength of a green molding sand depend upon the moisture of the sand at compaction; the lower the moisture content, the lower the hot and dry strengths of the sand. For example, a given percentage change in the amount of water has over five times the effect on sand strength as a similar percentage change in the amount of clay or other commonly employed green sand additive.
In general, foundry green molding sands consisting solely of sand, clay and water do not possess an optimum balance of properties. For this reason, a variety of additives have been employed in an effort to improve the properties of green molding sands. Typically, these additives are organic materials which are used as facing agents, expansion control agents and the like. In most cases, these organic additives are useful in improving only one property of the green sand and thus two or more additives may be required. In addition, an additive employed to improve one property frequently has an adverse effect on another property of the green sand mold. For example, sea coal or bituminous coal has been used as a facing agent, and while it does prevent burn-on, it has been found that increased amounts of clay and water are necessary to restore desirable physical properties possessed by the unmodified green sand.
The use of such organic additives is further limited because the total amount of materials which form gaseous materials under the elevated temperatures encountered during casting (i.e., water and organic additives) must be kept below 10 weight percent, based upon the weight of the sand. Excessive amounts of organic materials lead to the generation of more gas than can be dissipated by permeation through the mold body, and would lead to the failure of the mold and the generation of defects in the casting. Normally, the loss of weight on ignition due to volatilization of organic additives should be below about 7 weight percent, and preferably below about 4 weight percent.
Successful attempts to provide efficacious additives to molding sand include U.S. Pat. No. 3,962,550 which discloses that acenaphthylene (HY-PEEL.RTM.) is effective as a facing agent for molding sand, and U.S. Pat. No. 4,131,476 which discloses that salts of lithium or magnesium and an organic carboxylic acid, e.g., magnesium acetate (TAME.RTM.), are effective in improving the properties of hot green molding sand. Unfortunately, neither of these patents discloses products that can act alone to sufficiently improve the properties of green molding sand so as to negate the need for any other additives.
One successful approach at providing a useful and multifaceted additive to solve many of the problems associated with the use of green molding sands is disclosed in U.S. Pat. No. 3,816,145. The additive compound, trihydroxydiphenyl, marketed by Whitehead Brothers Co. as the product "DIKO.RTM." for use as disclosed in the patent, has proven quite effective in improving the properties of green molding sand. However, resorcinol, the raw material used for the manufacture of the trihydroxydiphenyl which has previously been considered a waste material, has become increasingly unavailable and expensive due to the discovery of other uses for it. Thus, trihydroxydiphenyl is no longer available in sufficient quantity and at sufficiently low price to be of practical value in the foundry industry.